Methods for transmitting a frame in a multi-user based wireless communication system

ABSTRACT

The present invention relates to methods for transmitting a frame by grouping a plurality of wireless terminals in a multiuser based wireless communication system. The present invention includes a method for transmitting a frame by including information about multiple receiver terminals in a MAC frame, another method for transmitting a frame by allocating group addresses and then using a corresponding group address for frame transmission, and yet another method for transmitting a frame by using a unique group sequence number that is allocated to each group and group control information that contains bitmap information. The group control information is included in a destination address field of a MAC layer, or in a signal field of a physical layer, or in both the MAC layer and the physical layer.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/478,069 filed on May 22, 2012, which is a by-pass continuation ofPCT/KR2010/008351 filed on Nov. 24, 2010, which claims priority toKorean Patent Application Nos. 10-2010-0022525 filed on Mar. 12, 2010and 10-2009-0113869 filed on Nov. 24, 2009, the disclosures of which arehereby incorporated in their entirety by reference.

TECHNICAL FIELD

Exemplary embodiments of the present invention relate a method fortransmitting a frame by grouping a plurality of wireless stations; and,more particularly, to a method for transmitting a frame to improvetransmission efficiency for grouping a plurality of wireless stations ina Multi-User Multiple Input Multiple Output (MU-MIMO) based wirelesscommunication system.

BACKGROUND ART

A wireless local area network (WLAN) basically supports an access point(AP) in a distributed system (DS) and a basic service set (BSS)including a plurality of wireless stations, which are not an accesspoint.

A Medium Access Control (MAC) protocol of a WLAN operates based on aCarrier Sense Multiple Access/Collision Avoidance (CSMA/CA).Accordingly, the WLAN disadvantageously wastes resources in channelcontention. In order to overcome such a problem, an enhanced MACprotocol defined in IEEE 802.11a introduced a burst transmission scheme.In the burst transmission scheme, multiple MPDUs are transmitted at aninterval of Short Inter-Frame Space (SIFS) during a TransmissionOpportunity (TXOP) time after obtaining a transmission right of awireless resource and ACKs are received in response to the transmittedMPDUs.

Further, Aggregated MSDU (A-MSDU) and Aggregated MPDU (A-MPDU) aredefined in IEEE 802.11 n. More than one MSDUs are tied together withoutan Inter Frame Space (IFS) and transmitted using one time wirelessresource through contention.

Lately, WLAN users have been abruptly increased. IEEE 802.11 ac has beenstandardizing a Very High Throughput (VHT) WLAN system in order toincrease data throughput provided by one BSS.

A VHT WLAN system supports, in a multiple (three)-STA environmentconsisting of one AP and two STAs, maximum throughput of 1 Gbps at theMAC SAP of the AP, as well as maximum throughput of 500 Mbps at the MACSAP of a wireless STA for point-to-point environments. There is alsoconsideration made so that each AP and STA of the VHT WLANsimultaneously supports compatibility with existing WLAN (IEEE 802.11a/nsystem).

Meanwhile, when a wireless STA acquires a TXOP in a wirelesscommunication system (e.g. WLAN), the STA needs to receive a responsethrough a response frame regarding a request frame in order to improvereliability of wireless communication. For example, frames requiringresponses include a RTS (Request-To-Send) frame, a BAR (Block ACKRequest) frame, a data frame, and frames for various requests (e.g.probe request, authentication request, association request). Responseframes responding to the frames requiring responses are as follows: aresponse frame responding to a RTS frame is a CTS (Clear-To-Send) frame;a response frame responding to a BAR frame is a BA (Block ACK) frame; aresponse frame responding to a data frame is an ACK or BA frame; andresponse frames responding to various request frames are frames ofvarious responses (e.g. probe response, authentication response,association response).

The responses are classified into immediate responses and delayedresponses. Responses (ACK frames) to a single piece of data correspondto the immediate responses; and block responses responding to acontinuous transmission or aggregated MPDU correspond to both immediateand delayed responses.

An immediate response is used in the following manner: whenPHY-RXEND.primitive of a received request frame is generated, a responseframe is transmitted after SIFS so that other wireless STAs do nottransmit. In this case, the frame exchange sequence constitutes a pair,as described above. The generated response frame may not include atransmission address.

On the other hand, a delayed response is used in the following manner: aresponse is made through an ACK frame as a basic response for informingof whether an initially generated request frame has been received ornot, and a response frame including requested information is thentransmitted. The response frame in this case may be transmitted throughEDCA (Enhanced Distributed Channel Access) regarding channels,piggybacked by another frame, or aggregated and transmitted togetherwith another frame. According to the delayed response scheme, atransmitting STA receives a response frame from a receiving STA and theninforms the receiving STA that the response frame has been receivedusing an ACK frame.

The request and response frame exchange sequence in such a WLAN may beapplied to a multiple user wireless communication system. When uplinkMU-MIMO technology is supported, wireless STAs can receive framesaccording to the above-mentioned frame exchange sequence andsimultaneously transmit response frames after IFS.

When uplink MU-MIMO technology is not supported, or when betterthroughput is desired even if uplink MU-MIMO technology is supported, awireless STA, after receiving a frame, needs to transmit a responseframe using a difference of channel or time.

Schemes for exchanging frames using a difference of time but the samebandwidth in a WLAN system include a scheme of exchanging a requestframe eliciting response frames and a response frame through channelaccess for each wireless STA, and a scheme of transmitting, by aplurality of wireless STAs, subsequent response frames through a singlerequest frame.

To be specific, the scheme of exchanging a request frame elicitingresponse frames and a response frame through channel access for eachwireless STA is as follows: an AP transmits a request frame to a firstwireless STA through channel access; the first wireless STA transmit aresponse frame after SIFS; the AP transmits another request frame to asecond wireless STA through channel access; and the second wireless STAtransmits a response frame after SIFS.

The scheme of transmitting, by a plurality of wireless STAs, subsequentresponse frames through a single request frame is as follows: a singlerequest frame including address information regarding multiple users istransmitted through channel access, and the wireless STAs thensubsequently transmit response frames after SIFS.

IEEE 802.11 standards recommend that, in order to avoid collisionbetween frames on a wireless path, control frames are transmitted to bereceivable by all STAs belonging to the corresponding BSS. In aMU-MIMO-based VHT WLAN system, control frames need to be transmitted tobe receivable by all STAs, even if a plurality of communication pathsare used. The control frames in this connection include RTS, CTS, ACK,BAR, BA, and various poll frames.

When a plurality of communication paths are used simultaneously, STAsreceiving control frames may have one-to-one correspondence torespective communication paths, meaning that that a control frame mustbe transmitted through each communication path. However, this increasesthe rate of overhead concerning transmission/reception of controlframes, degrading the advantage of using MU-MIMO technology.

DISCLOSURE Technical Problem

An embodiment of the present invention is directed to a method oftransmitting a frame by grouping a plurality of wireless stations inorder to effectively transmit a control frame and a data frame in awireless communication system using a multi-user multiple input multipleoutput (MU-MIMO) technology.

Other objects and advantages of the present invention can be understoodby the following description, and become apparent with reference to theembodiments of the present invention. Also, it is obvious to thoseskilled in the art to which the present invention pertains that theobjects and advantages of the present invention can be realized by themeans as claimed and combinations thereof.

Technical Solution

In accordance with an embodiment of the present invention, a method fortransmitting a frame to wireless stations in a wireless communicationsystem using a multi-user multiple input multiple output technology,includes: transmitting a group address registration request frame forgrouping the plurality of wireless stations; grouping the plurality ofwireless stations by receiving a group address registration responseframe in response to the group address registration request frame; andtransmitting a frame including the group address and receiving aresponse frame in response to the frame.

The response frame for the frame including the group address may besequentially received from each wireless station.

The group address request frame may include a group address, group dataQoS information, and group configuration information of wirelessstations forming a group.

The group configuration information may include addresses of thewireless stations forming a group, and addresses of the wirelessstations are a MAC address or an association identifier (ID).

In accordance with another embodiment of the present invention, a methodfor transmitting a frame to wireless stations in a wirelesscommunication system using a multi-user multiple input multiple outputtechnology, includes: transmitting a group control frame including groupcontrol information for grouping the plurality of wireless stations;grouping the plurality of wireless stations by receiving a responseframe for the group control frame; and transmitting a frame includingthe group control information and receiving a response frame to theframe.

The group control frame may include at least one of a group sequencenumber uniquely assigned to each group, bitmap information denotingwhether each one of wireless stations of a group receive a frame or not,and stream information related to multiple antennas. The group controlframe may further include receiving station information corresponding toa number of wireless stations forming a group.

The group control frame may further include receiving stationinformation corresponding to a number of wireless stations forming agroup and information on the number of wireless stations forming thegroup.

The method may further include updating the group control informationaccording to network configuration information such asconnection/cancelation or channel information received from eachwireless station.

The updated group control information may be included in a CTS-to-Selfframe and transmitted.

The updated group control information may be included in a control frameof a MAC level, which can be decoded by all wireless stations includinga legacy wireless station.

The updated group control information includes information on whether awireless station receives at least one group information and streaminformation related to multiple antennas and is transmitted to eachwireless station as a management frame form rough a broadcasting schemeor a unicast scheme.

Advantageous Effects

In accordance with an embodiment of the present invention, a pluralityof wireless stations are grouped by allocating a unique group address ora group sequence number through negotiation of a plurality of wirelessstations, and frames are transmitted to a plurality of wireless stationsusing the group address or the group sequence number. Accordingly, frametransmission efficiency is significantly improved in a MU-MIMO system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a MAC frame format where a firstembodiment of the present invention is applied to.

FIG. 2 is a diagram illustrating multiple receiving station informationin accordance with a first embodiment of the present invention.

FIG. 3 is a diagram illustrating a frame transmission procedure byallocating a group address and using a group address in accordance witha second embodiment of the present invention.

FIG. 4 is a diagram illustrating grouping information included in agroup address registration request frame in accordance with a secondembodiment of the present invention.

FIG. 5 is a diagram illustrating a format of a group control frame inaccordance with a third embodiment of the present invention.

FIG. 6 is a diagram illustrating overall operation of a wireless localarea network using a group number in accordance with an embodiment ofthe present invention.

FIG. 7 is a diagram illustrating a method for transmitting a groupcontrol information frame in accordance with a third embodiment of thepresent invention.

FIG. 8 is a diagram illustrating a frame transmission method inaccordance with an embodiment of the present invention.

FIG. 9 is a diagram illustrating a frame transmission method when groupcontrol information is included in a signal field of a physical layer inaccordance with an embodiment of the present invention.

BEST MODE FOR THE INVENTION

Exemplary embodiments of the present invention will be described belowin more detail with reference to the accompanying drawings. The presentinvention may, however, be embodied in different forms and should not beconstructed as limited to the embodiments set forth herein. Rather,these embodiments are provided so that this disclosure will be thoroughand complete, and will fully convey the scope of the present inventionto those skilled in the art. Throughout the disclosure, like referencenumerals refer to like parts throughout the various figures andembodiments of the present invention.

Hereinafter, a Multi-User Multiple Input Multiple Output (MU-MIMO)wireless communication system in accordance with an embodiment of thepresent invention is described based on an IEEE 802.11 wirelesscommunication system among various wireless communication systems.However, the present invention is not limited thereto. Embodiments ofthe present invention may be applied to all wireless communicationenvironments using the MU-MIMO technology.

A frame transferred using the MU-MIMO technology must have informationon receiving stations of multiple users. As a method for includinginformation on multiple receiving stations in a frame, the presentinvention introduces a method for including a list of addresses ofmultiple users in a transmitted frame as a first embodiment, a methodfor including a group address inside a transmitted frame as a secondembodiment, and a method of including group control informationincluding on a group sequence number and a bitmap inside a transmittedframe as a third embodiment. Here, the group sequence number and thebitmap are information for determine whether a group or a wirelessstation receives a corresponding frame or not.

At first, a method for including a list of addresses of multiple usersin a transmitted frame will be described as a first embodiment of thepresent invention.

FIG. 1 is a diagram illustrating a MAC frame format whether a firstembodiment of the present invention is applied to.

A frame control information field 101 includes a protocol field forreceiving a protocol field such as an IEEE 802.11 MAC version, a typeand sub-type field for identifying a type of a frame, and various fieldsfor storing various parameters for controlling a frame, such as a ToDSfield, a FromDS field, an additional fragment field, a retry field, apower management field, an additional data field, a WEP field, and asequence field.

A duration field 102 is used as one of frame transmitted during aNetwork Allocation Vector (NAV) and a Contention Free Period (CFP) and aPS-Poll frame.

A receiving station address (RA) field 103 is a field for storing areceiving destination of a frame, and a transmission station address(TA) field 104 is a field for storing a source transmitting acorresponding frame.

A sequence control information field 105 is used for fragmentationreassembly and dumping overlapped frame. The sequence controlinformation field 105 includes a 4-bit fragment number field and a12-bit sequence number.

A frame body field 108 is a data field for storing a payload. A FrameCheck Sequence (FCS) field 109 is used to check integrity of a framereceived from a predetermined station.

Among the fields of the MAC frame, essential fields are the framecontrol information field 101, the duration field 102, the receivingstation address (RA) field 103, and the FCS field 109. The remainingfields may be omitted or added according to a type of a frame in orderto save a wireless resource. For example, in case of a data frame notusing QoS, a QoS control information field 106 is excluded. Further incase of an IEEE 802.11n dedicated frame, a HT control information fieldis added with a QoS field to support a HT function.

The frame control information field 101 must be analyzed by all stationsin order to inform a type of a received frame. The duration field 102must be analyzed by all stations to prevent collision between frames ona wireless medium. The receiving station address (RA) field 103 must beanalyzed by all stations to confirm whether a destination address of areceived frame is an own address or not. If the destination address ofthe received frame is own address, the receiving station decodesremaining information in a corresponding frame.

Therefore, it is required to include a multiple reception indicator inone of fields that must be analyzed by all stations in order toconfigure a frame including information on a plurality of receivingstations in one frame. Referring to FIG. 1, as the multiple receivingindicator for indicating that a received frame includes information on aplurality of users, a predetermined bit of the frame control informationfield 101 or the receiving station address field.

The multiple receiving station information 107 is added after a headerincluding frame control information for sustaining compatibility with aframe of an existing WLAN system. Referring to FIG. 1, the multiplereceiving station information 107 is added between the QoS controlinformation field 106 and the frame body 108 or included in the framebody 108 as a predetermined constituent element. Therefore, wirelessstations not capable of processing multiple receiving stationinformation detect the multiple receiving station information as a framebody.

FIG. 2 is a diagram illustrating multiple receiving station informationin accordance with an embodiment of the present invention.

As shown in FIG. 2, the multiple receiving station information 107includes information on the number of receiving stations 201 andinformation on each receiving station as many as the number of receivingstations. Here, the receiving station information 202 includesinformation on an address of a receiving station. The receiving stationinformation 202 further includes QoS information and modulation codingscheme (MCS) information selectively.

The multiple receiving station number 201 is a selective element thatcan be included or not included in the multiple receiving stationinformation. The receiving station address may be a MAC address or anassociation ID.

Meanwhile, in case of a control frame having a fixed format such as aRTS frame or a BAR frame, an extension format such as a Control WrapperFrame defined in IEEE 802.11n MAC protocol can be used. If such anextension format is not used, it may be defined as a new control frameand used.

A method of receiving and processing a frame including multiplereceiving station information in accordance with an embodiment of thepresent invention will be described.

At first, a case of using a predetermined bit of frame controlinformation as a multiple receiving indicator will be described.

When a multiple receiving indicator is setup in frame controlinformation, receiving stations addresses are analyzed according to thenumber of receiving stations. If own address is included in thereceiving station addresses, it is determined that a received frame is aframe transmitted to itself. However, if an own address is not in thereceiving station addresses, it is determined that the received frame isnot a frame transmitted to itself. When the multiple receiving stationnumber information is not included in the received frame, a step ofanalyzing the multiple receiving station number information may beomitted from a receiving procedure.

Meanwhile, if the multiple receiving indicator is not setup in framecontrol information, the multiple receiving station information is notincluded in the received frame. Accordingly, a destination address of areceived frame is analyzed. If the destination address is an ownaddress, it is determined that the received frame is a frame transmittedto itself. If not, it is determined that the received frame is not aframe transmitted to itself.

A case of using predetermined destination address information as amultiple receiving indicator be described.

When destination address information of a received frame is apredetermined value assigned as a multiple receiving indicator,addresses of receiving stations are analyzed according to the number ofmultiple receiving stations. If an own address is included in receivingstation addresses, it is determined that a received frame is a frametransmitted to itself. If not, it is determined that a received frame isnot a frame transmitted to itself.

Meanwhile, if the destination address information of a received frame isnot the predetermined value assigned as the multiple receivingindicator, it is determined as a typical destination address.Accordingly, if the destination address is an own address, it isdetermined that a received frame is a frame transmitted to itself. Ifnot, it is determined that the received frame is not a frame transmittedto itself.

In order to transmit a frame using a group address, it is required togroup wireless stations in a basic service set (BSS).

FIG. 3 is a diagram illustrating a procedure of grouping a plurality ofstations and transmitting a frame to grouped stations in accordance withan embodiment of the present invention.

At first, an access point (AP) performs a group address negotiationprocedure for negotiating a group address with wireless stations in abasic service set (BSS). Referring to FIG. 3, the access point (AP)performs a group address negotiation procedure with a first wirelessstation STA1 and performs a group address negotiation procedure with asecond wireless station STA2.

As shown in FIG. 4, a frame used for the group address negotiationprocedure includes a group address 401, group data QoS information 402,and group configuration information 403. Here, the group configurationinformation includes information on addresses of stations in apredetermined group and timing information such as a responsetransmission time in case of a delayed response.

In the group address negotiation procedure, a response frame may betransmitted using an immediate response scheme or a delayed respondscheme. FIG. 3 illustrates a group address negotiation proceduretransmitting a response frame using a delayed response scheme. Forexample, request and response frames for the group address negotiationprocedure are transmitted as a management frame.

Although it is not shown in the drawings, a method of transmitting aresponse frame in response to a group address registration request usingan immediate response method will be described.

An access point (AP) transmitting a group address registration requestframe transmits a group address registration request frame to a firstwireless station. The first wireless station transmits a group addressregistration response frame in response to the group addressregistration request frame after a short inter-frame space (SIFS).Continuously, the access point (AP) transmits a group addressregistration request frame to a second wireless station. The secondwireless station transmits a group address registration response frameafter a SIFS in response to a group address registration request frame.Through the above described procedure, the access point (AP) transmits agroup address registration request frame individually to a plurality ofwireless stations and receives a response frame in response to the groupaddress registration request frame. The described procedure may beapplied when request and response frames are transmitted as a controlframe in a group address negotiation procedure.

Hereinafter, a group address negotiation procedure using a delayedresponse method will be described with reference to FIG. 3.

At step S301, the access point (AP) transmits a group addressregistration request frame to a first wireless station STA1. At stepS302, the access point (AP) receives an ACK frame from the firstwireless station STA1 after a SIFS. At step S303, the first wirelessstation STA1 performs channel access and transmits a group addressregistration response frame for the group address registration request,to the access point (AP). At step S304, the access point (AP) receivesthe group address registration response frame and transmits the ACKframe to the first wireless station STA1 after a SIFS.

At step S305, the access point (AP) transmits a group addressregistration request frame to a second wireless station (STA2). At stepS306, the access point (AP) receives an ACK frame from the secondwireless station (STA2) after the SIFS. At step S307, the secondwireless station (STA2) performs channel access and transmits a groupaddress registration response frame for the group address registrationrequest to the access point (AP). At step S308, the access point (AP)receives the group address registration response frame and transmits anACK frame to the second wireless station (STA2) after a SIFS.

Through the above described procedure, the access point (AP) performsthe group address negotiation procedure for negotiating a group addresswith a plurality of wireless stations. Here, the group addressregistration frame may include agreement and refusal to the groupaddress registration request.

At step S309, the access point (AP) transmits a RTS frame to a pluralityof wireless stations using a group address after completing grouping aplurality of stations through the group address negotiation procedure.The first and second wireless stations receiving the RTS framesequentially transmit a CTS frame according to a response timecalculated through a transmission MCS of a frame and a response sequenceincluded in the RTS frame at steps S310 and S311.

At step S312, the access point (AP) transmits a data frame in a unit ofa MU-MIMO aggregated MPDU (A-MPDU) to grouped wireless stations. Atsteps S313 and S314, the access point (AP) receives a Block ACK frameeach wireless station.

Here, a plurality of group addresses may be allocated to one wirelessstation. That is, grouped wireless stations may have more than one groupaddress.

Hereinafter, a method of including group control information includingon a group sequence number and a bitmap inside a transmitted frame as athird embodiment will be described.

FIG. 5 is a diagram illustrating a group control frame format fornegotiating group control information in accordance with an embodimentof the present invention.

The group control information includes the number of multiple receivingstations 501, a group sequence number 502 unequally allocated to onegroup, STA bitmap information 503 for informing whether a frame isreceived from wireless stations or not, and stream information 504. Themultiple receiving station information 505 includes information onreceiving stations corresponding to the number of multiple receivingstations.

The information 501 on the number of the multiple receiving stationsshows the number of stations assigned as one group. It is an optionalelement, not an essential element.

The group sequence number 502 is a unique number for managed by eachaccess point for grouping wireless stations in a basic service set(BSS). For example, the group sequence number 502 may have a six-bitlong. If a group number from 1 to 64 can be expressed by combiningbitmap information and stream information, a group sequence number maybe omitted. In other words, a group unique number may be expressed as alocation of more than one group control information combined of bitmapinformation and stream information. When a group sequence number is usedin a transmitted frame, a transmission frame includes group controlinformation such as bitmap information and stream information located ata location matching with a group sequence number.

The STA bitmap information 503 of a wireless station is informationdenoting whether a frame is received at each receiving station accordingto a sequence of n receiving station information fields. For example,when bitmap information is “1101” if the number of multiple receivingstations is 4 (n=4), it means that the frame is transmitted to a firstwireless station, a second wireless station, a third wireless station,and a fourth wireless station. Accordingly, each receiving stationrecognizes an own location from a bitmap using an address in a receivingstation information field while being allocated with group controlinformation.

Meanwhile, because the number of wireless stations corresponding to acorresponding group sequence number is 1 in case of informing a groupsequence number of an own group to one of wireless stations, one-bitinformation is used to inform whether a frame is received or not as avalue of 1 or 0. Also, one station may be included in a plurality ofgroups. In case of expressing a plurality of groups using a groupsequence number, the bitmap information of the wireless station may beused to inform whether a frame is received from a corresponding group.

The stream information 501 is information for denoting a stream locationof a transmission antenna in a MU-MIMO environment. That is, the streaminformation 504 is information denoting which antenna is used totransmit a frame among four antennas. Accordingly, a location of streamto receive at a receiving station can be determined through the streaminformation. Further, the stream information may include individualinformation for a streams where a is an integer number. When a streamsare classified into b groups, a value b may denote a receiving location.

The receiving station information 505 includes information on an addressof a receiving station. The receiving station information 505 furtherinclude QoS information and modulation coding scheme (MCS) informationas well as receiving station address information. The receiving stationaddress may be one of a MAC address and an association ID.

Such a group control frame may be transmitted to a plurality of wirelessstations using a broadcasting method. Also, the group control frame maybe transmitted to each wireless station using a unicast method. In caseof using the unicast method, a group control frame only includes a groupsequence number 502, own bitmap information 503, and stream information504.

Frames transmitted after completing allocating the group controlinformation for a plurality of wireless stations include only a groupsequence number and bitmap information and stream information of awireless station. Accordingly, each wireless station recognizes adestination of a frame using a group sequence number and bitmapinformation allocated to itself and receives a signal using streaminformation.

Meanwhile, one of wireless stations can be allocated with more than onegroup control information. That is, a group control frame may includebitmap information and stream information of a wireless stationcorresponding to more than one group sequence number. The same frameformat shown in FIG. 5 may be used when a wireless station is allocatedwith a plurality of group control information.

FIG. 6 is a diagram illustrating overall operation of a wireless localarea network (WLAN) using a group number in accordance with anembodiment of the present invention.

The overall operation of a WLAN in case of using a group numberincludes: defining, negotiating, and allocating a group number,requesting and responding sounding information, transmitting MU-MIMOdata, and responding the MU-MIMO data. Here, exchanging of a RTS/CTSframe may be performed at a time of requiring channel protection likeprior to sounding or MU-MIMO data transmission step.

Here, if the defining, negotiating, and allocating a group number andthe sounding are performed at the same time, a frame transmitted forthose steps must have at least one purpose. It may be ineffective in aview of control and loss of operation. Accordingly, it is prefer toperform the group number defining, negotiating, and allocating step andthe sounding step independently. In an embodiment of the presentinvention, the group number defining, negotiating, and allocating stepis performed independently from the sounding step when it is required togroup a plurality of wireless stations in a basic service set (BSS).

FIG. 6 is a diagram illustrating overall operation of a WLAN systemhaving two groups by assigning one group using association IDs (AID) offour receiving stations.

Referring to FIG. 6, a group number such as a group ID or a groupsequence number is defined, a group number is assigned to each stationthrough negotiation or a group number is allocated to each station, andsounding is required using the group number at step S601.

At step S602, each wireless station transmits an ACK frame for thesounding request after a SIFS based on a predetermined schedule, therebyperforming feedback to the sounding request.

After receiving the ACK frame for the sounding request from the lastwireless station, an access point (AP) transmits a CTS-to-Self frame asa NAV distribution step after a SIFS before transmitting a MU-MIMO dataframe at step S603. Here, the CTS-to-Self frame includes group controlinformation updated based on feedback information reduced in a previoussounding step. Here, the group control information is updated accordingto channel information of each wireless station included in backupinformation or network configuration information such asconnection/connection cancel information.

As described above, the CTS-to-Self frame informs a response sequence toa receiving MAC including wireless stations not participated forreceiving, and a wireless station to receive data calculate atransmission time of a response frame using the response sequenceinformation.

After completing NAV distribution, the access point transmits a UM-MIMOdata frame including a group number to a plurality of wireless stationsand receives a response frame such as an ACK frame in response to theUM-MIMO data frame from each wireless station.

Meanwhile, the updated group control information may be transmitted bybeing included not only in the CTS-to-Self frame but also in a MAC levelcontrol frame that can be included in a legacy wireless station anddecoded by all wireless stations. The updated group control informationincludes information on whether a predetermined wireless stationreceives more than one different group information and streaminformation. Such a updated group control information is transmitted asa broadcasting scheme or a unicast scheme as a management frame form.

Transmitting a group control frame to a plurality of wireless stationsin a broadcasting scheme will be described.

Stations receiving a group control frame determine that the groupcontrol fame s a frame transmitted to address if a corresponding bit ina bitmap of a wireless station is setup as ‘1’.

The wireless stations transmit an ACK frame for the group control frameafter a SIFS. Here, each wireless station sequentially transmits the ACKframe.

In a method of sequentially performing response at each wireless stationin accordance with an embodiment of the present invention, each wirelessstation transmits an ACK frame after a time calculated by multiplyingthe number of bits setup as 1 among bits prior to an own bit in a bitmapof a wireless station by the sum of an ACK frame transmission time and aSIFS. In other words, a wireless station transmits an ACK frame afterall of wireless stations having I as a bit value of a bitmap before itsown turn.

FIG. 7 is a diagram illustrating a method for transmitting a groupcontrol frame in accordance with an embodiment of the present invention.

Referring to FIG. 7, the number of wireless stations forming a group is4 and a bitmap is formed of 4-bits. An access point (AP) creates a groupcontrol frame including a group sequence number, bitmap information of awireless station, stream information, and receiving station information.The access point (AP) sets up a NAV protection value in order to safelyreceive an ACK frame from multiple users formed as four wirelessstations.

Referring to FIG. 7, each wireless station transmits an ACK frame at itsown turn, thereby performing response to a group control frame. Here,the access point sets up a corresponding bit of a bitmap as ‘1’ for awireless station performing response at a corresponding turn when thewireless station transmits a MU-MIMO data frame. However, the accesspoint sets up a corresponding bit of a bitmap as ‘0’ for a wirelessstation not performing response when the wireless transmits a MU-MIMOdata frame.

FIG. 8 is a diagram illustrating a method for transmitting a frame usinggroup control information in accordance with an embodiment of thepresent invention.

A frame transmission method of a typical WLAN generally includesRTS/CM/DATA/ACK steps. In case of transmitting Mu-MIMO data frame, it ayinclude the same steps.

The exchange of a RTS/CTS frame is a function required for a hidden-nodeproblem and for channel protection for a frame longer than a RTSthreshold. In case of transmitting data using a MU-MIMO technology, aprocess of exchanging a RTS/CTS frame is required. Here, a RTS frameincludes group control information in a receiving station address field,Here, the group control information includes at least a group sequencenumber and bitmap information. When the group control information isrecorded at the receiving station address field, it is possible totransmit a frame to multiple users without modifying an existing frameformat or without adding additional frame.

Wireless stations having the same group sequence number transmit a CTSframe for a RTS frame at an own response turn.

After exchanging the RTS/CTS frame, the access point (AP) transmits theCTS-to-Self frame, The CTS-to-Self frame may include updated groupcontrol information,

The access point transmits a MU-MIMO data frame using the updated groupcontrol information. Referring to FIG. 8, four wireless stations form agroup included in the group control information. After updating groupcontrol information, three wireless stations receive a MU-MIMO dataframe transmitted using a corresponding group sequence number. That is,a bitmap is configured as ‘1101’ by setting up a bit for the thirdwireless station as ‘0’ in the bitmap. Accordingly, it is possible toindicate there is no data transmitted to the third wireless station.

Each wireless station sequentially transmits a Block Ack frame for adata frame using the updated group control information included in theCTS-to-Self.

Since MAC addresses of corresponding wireless stations are included indestination addresses of each A-MPDU of the MU-MIMO data frame, data arereceived from multiple users independently from each beamformed A-MPDU.Here, grouping wireless stations to receive data using MU-MIMO isdecided by a combination providing superior throughput when data framesare transmitted at the same time using MU-MIMO and a combination of QoSinformation of data to be transmitted,

Until now, the frame transmission method when group control informationis included in a MAC frame was described. In case of transmitting aframe by including the group control information in a MAC frame, a stepof transmitting a CTS-to-Self frame is performed prior to a step oftransmitting a MU-MIMO data frame. That is, accurate stream informationcan be transferred by performing the CTS-to-Self frame transmission stepprior to the MU-MI MO data frame transmission step. Accordingly, thereis an overhead of a MAC layer generated.

Therefore, an embodiment of the present invention includes a method forincluding group control information in a physical layer signal field.

FIG. 9 is a diagram illustrating a frame transmission method when aphysical layer signal field includes group control information.

In case of including the group control information in the physical layersignal field, the group control information may include a group sequencenumber and stream information. Since each receiving station address of aMAC frame includes each receiving station address, bitmap information isnot required to be included in the physical layer signal field. In otherwords, each wireless station grouped using a physical layer signal fielddetermines whether a frame is received or not. When a wireless stationdetermines that a received frame is a frame transmitted to itself, eachwireless station confirms a destination address through a MAC frame.Accordingly, it is not required to include bitmap information in aphysical layer signal field.

Due to a compatibility problem with a legacy wireless station, groupcontrol information cannot be included in a physical layer signal fieldof a control frame such as a RTS frame, a CTS frame, a BAR frame, and aBA frame sustaining a legacy frame. The group control information may beincluded in only a physical layer signal field of a MU-MIMO data frame.

Referring to FIG. 9, all of VHT wireless stations decode a physicallayer signal field of a MU-MIMO data frame and interrupt a receivingoperation when it is not an own group number according to correspondingdecoded group control information. However, VHT wireless stations decodeonly corresponding streams using stream information when it is the owngroup number.

In FIG. 9, a Req/Rsp frame includes a RTS frame modified for a scheduledCTS frame and a CTS frame which is a response for the modified RTSframe. On the contrary, the Req/Rsp frame includes a sounding requestframe including group control information and a channel feedbackresponse frame if the sounding procedure is included,

In case of transmitting a data frame by including group controlinformation in a physical layer signal field, the RTS/CTS step isrequired to modified. Since legacy wireless stations cannot perform NAVupdate using beamformed VHT data, a robust channel protection method isrequired.

Hereinafter, a method for including group control information in a MAClayer and a method for including group control information in a physicallayer (PHY).

Meanwhile, a frame can be transmitted by including group controlinformation at both of a MAC layer and a PHY layer. In this case, thegroup control information is included in the CTS-to-Self frame of a MAClayer, and the group control information is also included in a PHYsignal field of a VHT data frame at the same time.

A method of transmitting a frame by including the group controlinformation in the both of the MAC layer and the PHY layer can reduceadditional overhead and effectively support compatibility with a legacywireless station.

For example, an overhead for always transmitting a CTS-to-Self frameprior to a VHT data frame can be reduced because modified group controlinformation can be transferred using a PHY signal field of a VHT dataframe. A channel protection function can be supported without modifyinga legacy control frame format because a frame is transmitted byincluding the group control information in a legacy CTS-to-Self frame ora receiving station address field of a legacy RTS frame. That is, achannel protection function can be enhanced, and a transmission time ofeach multi-response frame can be recognized because the updated groupcontrol information is transferred to a MAC layer of a receivingstation. Accordingly, each receiving station can further accuratelyperform a related process.

The above-described methods can also be embodied as computer programs.Codes and code segments constituting the programs may be easilyconstrued by computer programmers skilled in the art to which theinvention pertains. Furthermore, the created programs may be stored incomputer-readable recording media or data storage media and may be reactout and executed by the computers. Examples of the computer-readablerecording media include any computer-readable recoding media, e.g.,intangible media such as carrier waves, as well as tangible media suchas CD or DVD.

While the present invention has been described with respect to thespecific embodiments, it will be apparent to those skilled in the artthat various changes and modifications may be made without departingfrom the spirit and scope of the invention as defined in the followingclaims.

INDUSTRIAL APPLICABILITY

Embodiments of the present invention are applied to a MU-MIMO WLANsystem that groups a plurality of wireless stations and transmits aframe.

1. A method of wireless communication, the method comprising: receiving,by a receiver, a first frame including first information and secondinformation, the first information indicating whether the receiver is amember of at least one of a plurality of groups; receiving, by thereceiver, a second frame, the second frame including a group identifier;determining, by the receiver, whether the second frame is for thereceiver based on the group identifier and the first information; anddetermining, by the receiver, a number of streams based on the secondinformation.
 2. An apparatus for a wireless terminal, comprising: amemory; and a processor coupled to the memory, wherein the processor,when executing instructions stored in the memory, is configured to causethe wireless terminal to: receive a first frame, the first frameincluding first information and second information, the firstinformation indicating whether the wireless terminal is a member of atleast one of a plurality of groups and the second frame includes a groupidentifier; and determine whether the second frame is for the wirelessterminal based on the group identifier and the first information and anumber of streams based on the second information.
 3. A method ofwireless communication, comprising: transmitting, to a receiver, a firstframe including first information and second information, the firstinformation indicating whether the receiver is a member of at least oneof a plurality of groups; and transmitting, to the receiver, a secondframe, the second frame including a group identifier, wherein the secondinformation allows the receiver to determine a number of streams.